Do not down-play the toxicity of this algae. Every year unfortunate dogs die because their owners were unaware of the severity of illness blue-green algae can invoke.
Cyanobacteria are often referred to as blue-green algae because of their appearance when they clump together. Unable to be seen by the naked eye, except when they get together en masse, forming visible colonies called blooms. These blooms are often viscous and slimy, appearing to us as greenish slimy paint or brownish floating clumps, sometimes green flakes or foam on the shore of a pond or lake. They are usually found in slow-moving bodies of water and mainly are a problem in the hot summer months when there is little rain.
The bacteria may be present in a harmful form even if you cannot see it, so take note of any warning signs in the area, like a pungent odor that resembles sewage or natural gas, or dead animals and fish at or near the shoreline.
Not all algae in the water are dangerous but if temperatures are above 75 and there has been little rain, conditions are good for toxic blooms to occur. You cannot tell just by looking if algae in the water is toxic or not. Often authorities will post signs as soon as a toxic bloom is identified so pay attention to signs, but do not assume it’s safe if you don’t see a warning posted.
Cyanobacteria can produce toxins called microcystins and anatoxins that are dangerous to pets, people and wildlife.
Microcystins can result in liver damage or failure. Signs of liver injury include vomiting, diarrhea, blood in stool or black, tarry stool, weakness, pale mucous membranes, jaundice, seizures, disorientation, coma, and shock.
Death can happen in days because of liver failure, without immediate critical care.
Anatoxins result in neurotoxicity. Signs include excessive tearing of the eyes or drooling, neurologic signs like tremors, muscle rigidity, paralysis, etc., blue discoloration of the skin and mucous membranes, and difficulty breathing.
Death can follow within minutes to hours of exposure because of respiratory paralysis!
There is no current antidote for these toxins. Immediate veterinary care is imperative. Treatment may include anti-seizure medication, oxygen, and other aggressive care by your veterinarian and time is of the essence.
If you have any question as to the safety of the water, you wish to occupy, please avoid it.
Sources: Blue Cross for Pets, ASPCA 2019, Pet Poison helpline